A centrifuge spins a test tube rapidly to sediment particles faster. In the rotating frame, this is described using the fictitious centrifugal force. What is really happening, described in an inertial frame?

1. In an inertial frame, the correct description is that the particles' own inertia carries them along a straight tangent line rather than being pushed outward by any real force. 2. The test tube itself is what is forced into a circular path, by the centripetal force supplied through the tube walls, while the denser particles resist that turning slightly more. 3. Nothing in this process removes mass from the particles, so that option misdescribes what a centrifuge does. 4. The fluid pressure inside the tube does not drop to zero, since the walls and fluid still exert normal contact forces throughout the spin; the entire effect is explained by inertia and the tube's forced circular path, not by any real outward force. _Source: OpenStax College Physics (CC BY 4.0), Ch 6 "Uniform Circular Motion and Gravitation", section 6.4 Fictitious Forces and Non-inertial Frames: The Coriolis Force_